ぶっ (bu) is just something you add to make certain verbs sound more aggressive, disorderly, or even vulgar. So bukkake just means splash aggressively. We have a dish called Bukkake-don, which means a bowl of rice that’s been splashed with a bunch of toppings. It’s just a normal word, just like how you can use “facial” in everyday context without sounding like a sex fiend in English.